1. Technical Field
This disclosure generally relates to an image forming apparatus that can execute both sides printing on a sheet which is transported by a transport belt by an electrostatic force.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile, a copying apparatus, and a multifunctional apparatus that has the above functions, for example, an inkjet recording apparatus is widely known. In the inkjet recording apparatus, a recording head (an image forming unit) composed of ink droplet ejecting heads which eject ink droplets of recording liquid is used, and an image is formed on a recording medium (sheet) by ejecting the ink droplets of the recording liquid onto the recording medium from the recording head while carrying the recording medium. In this, the recording medium is not limited to a sheet, and any other recording medium such as an image transferring medium can be used as the recording medium; and as the image forming, image recording, image printing, letter printing, letter transferring and so on are included.
When an image is formed by an inkjet recording apparatus, since ink is attached onto a sheet, a phenomenon in which the sheet is stretched by water contained in the ink occurs; this phenomenon is called cockling. By the cockling, the sheet has a wave shape, and a gap between the nozzle surface of the recording head and the surface of the sheet is changed among positions. When the cockling is large, the sheet may contact the nozzle surface so that the nozzle surface is stained and the sheet is also stained. That is, image quality is degraded, and the ejection position of the ink droplet on the sheet is shifted from the original target position.
In Patent Document 1, an image recording apparatus is disclosed. In the image forming apparatus, in order to maintain the flatness of a sheet, a transport belt of an endless type is used, the sheet is attached onto the transport belt by an electrostatic force by charging a voltage on the surface of the transport belt, and the sheet is transported by the rotation of the transport belt. With this, the sheet is prevented from being separated from the transport belt and high flatness of the sheet is maintained.
[Patent Document 1] Japanese Laid-Open Patent Application No. 2004-175490
In Patent Document 2, an image transferring sheet carrying apparatus is disclosed. In the image transferring sheet carrying apparatus, since a transferred image suffers a bad effect from cockling and curling of an image transferring sheet, the image transferring sheet is attached onto an image transferring position by an electrostatic force of an electrostatic attraction member, and image transferring operations on the image transferring sheet and moving operations of the image transferring sheet are executed. Further, a switching unit is included, that is, attraction or non-attraction of the image transferring sheet by the electrostatic attraction member is switched by the switching unit based on the type of image transferring sheet being used.
[Patent Document 2] Japanese Laid-Open Patent Application No. 2000-246981
In Patent Document 3, an inkjet recording apparatus which can execute both sides printing is disclosed. In the inkjet recording apparatus, a sheet is transported by a transport belt and an image is transferred on one side of the sheet; after this, at least a part of the sheet is output from the inkjet recording apparatus. After a time which is needed for drying ink is passed, the sheet is transported again to the inkjet recording apparatus by a switchback method.
[Patent Document 3] Japanese Laid-Open Patent Application No. 2000-001010
However, as described above, in a case where a sheet is transported by a transport belt to which an electric charge is applied by being attached thereto, when a part of the tip of the sheet is curled, since the part of the tip of the sheet does not contact the transport belt closely, the electrostatic force between the part of the tip of the sheet and the transport belt becomes small.
Especially, in a low humidity environment, in a case where both sides printing is executed on a sheet, when an image is formed on the first side of the sheet by ejecting recording liquid, curling of the sheet occurs due to rapidly absorbing water. The curling occurrence on the sheet is different in a relationship between the sheet carrying (transporting) direction and the grain direction of the sheet. In short grain carrying in which the sheet carrying direction crosses the grain direction of sheet, the tip and the end of the sheet contact the transport belt and the center part of the sheet is separated from the transport belt in the sheet carrying direction. In long grain carrying in which the sheet carrying direction is the same direction as the grain direction of sheet, both side edges of the sheet contact the transport belt and the center part of the sheet is separated from the transport belt in the direction which crosses the sheet carrying direction.
Therefore, in a case where the both sides printing is executed, when the sheet is transported to an image forming region in which the recording head is disposed so as to print an image on the second side of the sheet, a part of the sheet which is separated from the transport belt rubs the nozzle surface of the recording head due to the curling of the sheet. Consequently, the image on the sheet may be degraded and jamming may occur.